Vasopressin increases the permeability of the cell membrane to water and to a number of small solutes. There is a good deal of evidence that the hormone opens independent membrane pathways for water and solutes. Our most recent experiments show that the pathways for water and that for urea can be opened separately, by controlling the concentration of vasopressin, or the ratios of cyclic nucleotides to phosphodiesterase inhibitors used in stimulating the bladder. Using this ability to open the channels separately, we are currently studying the physical properties of the water and urea channels, and the cyclic nucleotide systems involved in their activation.